Field of the Invention
The present invention relates to solid-state image sensors and image-capturing devices that include solid-state image sensors.
Description of the Related Art
Nowadays, in order to reduce the size of digital cameras, there is a demand for solid-state image sensors that are compatible with camera lenses having a short back focus. In addition, in order to broaden the dynamic range of digital cameras, large-size solid-state image sensors are being used even in small-size digital cameras. In digital cameras that meet the aforementioned demands, light beams are incident on pixels in peripheral regions of the solid-state image sensors at large angles, and thus the light beams are incident on the pixels at positions that are shifted from photoelectric conversion units disposed in the pixels. Consequently, the utilization efficiency of the light beams are reduced, and the sensitivity of the pixels in the peripheral regions decreases.
To address such an issue, Japanese Patent Laid-Open No. 2010-182765 discloses a solid-state image sensor in which microlenses provided on the surface of the pixels are disposed so as to be shifted toward the center of the solid-state image sensor, and the solid-state image sensor is thus capable of detecting light beams incident on the pixels at an angle with high efficiency.
Meanwhile, techniques for detecting a focal point in digital cameras are known. With regard to such techniques, Japanese Patent Laid-Open No. 2009-158800 discloses a solid-state image sensor in which some of the pixels in the solid-state image sensor are provided with a configuration for detecting a focal point through a phase-difference method. In the phase-difference method, images of light rays that have passed through different regions in a pupil of an imaging optical system are compared, and the distance from the focal point to a target of imaging is determined by triangulation of a stereo image. With the technique disclosed in Japanese Patent Laid-Open No. 2009-158800, light beams that have passed through different regions in the pupil of the imaging optical system are coupled, by using microlenses, on interlayer films that are spaced apart from each other. The light beams that have been incident on the interlayer films that are spaced apart from each other are then guided to respective photoelectric conversion units, and the distance is determined on the basis of electric signals from the photoelectric conversion units.
Even with a solid-state image sensor such as the one disclosed in Japanese Patent Laid-Open No. 2009-158800, there arises a problem in that the sensitivity of pixels in the peripheral region decreases when the back focus is shortened or the dynamic range is broadened. To address such an issue, employing a technique in which the microlenses are disposed so as to be shifted toward the center of the solid-state image sensor, as disclosed in Japanese Patent Laid-Open No. 2010-182765, is considered.
However, if the technique disclosed in Japanese Patent Laid-Open No. 2010-182765 is merely applied to the technique disclosed in Japanese Patent Laid-Open No. 2009-158800, another problem as described below arises. Specifically, in a case in which a microlens is disposed so as to be shifted, angles at which principal rays enter respective waveguides differ from each other, and thus coupling efficiencies of the light beams incident on a pixel differ among the plurality of waveguides. Therefore, a difference is produced among the sensitivities of photoelectric conversion units corresponding to the respective waveguides in the pixel, and thus the accuracy in measuring the distance through the phase-difference method is deteriorated.
The present invention is directed to providing a solid-state image sensor that is capable of measuring a distance with high accuracy even in a peripheral region of the solid-state image sensor.